When the Justice Department on Friday formally dropped its four-year-old case against two former pro-Israel lobbyists for allegedly conspiring to violate the 1917 Espionage Act, prosecutors cited several reasons for their decision but did not provide details.

Some details from the point of view of the defendants, Steven J. Rosen and Keith Weissman, can be gleaned from a March 27 letter to Attorney General Eric H. Holder Jr., asking the Obama administration to review the case. That was written by the pair's lead defense attorneys, Abbe David Lowell, John N. Nassikas III and Baruch Weiss.

The two lobbyists had been charged in August 2005 with conspiring between 1999 and 2004 to disclose national defense information to people not authorized to receive it -- the first time that civilian, non-government employees had been prosecuted under the then-88-year-old act. The case had broad implications, because the same charges technically could be applied to academics, think tank analysts and journalists who seek and receive security information in conversations every day.

The government and the defendants filed many motions to determine what the law required for a finding of guilt and to clarify rules for presenting evidence and witnesses at trial. U.S. District Judge T.S. Ellis III presided over 40 hearings on the matter, and he delivered 12 published decisions. Seven separate trial dates were set and postponed during the past 3 1/2 years before the date of June 2 was established.

Prosecutors and investigators had used FBI wiretaps to pursue Rosen and Weissman for at least five years. The two were charged in 2005 with conspiring to obtain classified information and pass it to the Israeli government and journalists from The Washington Post and other news organizations. The first reason prosecutors gave on Friday for ending the case was the likelihood that classified information would be revealed and concern about "damage to the national security that might result."

The defense attorneys wrote that Ellis's rulings permitted them to introduce "a good deal of classified material." But the classified documents they planned to use would show that the information orally passed to the defendants was similar to, but only part of, more sensitive classified information.

The lawyers said it was "ironic" that to prove "the sanctity of alleged national defense information, the prosecution will risk the disclosure of classified documents . . . the defendants never saw."

The second reason prosecutors gave for ending the case was a question of whether the government would prevail at trial.

The defendants' attorneys wrote that they would show that the information relayed to their clients was not classified defense information but material already in the public domain and "not potentially damaging to national security."

To demonstrate that, the lawyers wrote that two of the government officials who prosecutors said passed classified information to the defendants "have told both us and/or government investigators, that they were authorized to speak with our clients and knew full well (and even intended) that our clients pass the information on to others."

In addition, the lawyers said that Rosen and Weissman were under government surveillance, including telephone wiretaps, for five years, from 1999 to 2004, yet over that time nothing was done to stop them. Eventually, according to the lawyers, Lawrence Franklin, a Defense Department Iran expert who himself was under investigation for leaking information, was turned into a cooperating informant.

Franklin, wearing a recording device, met with Weissman and "induced him into believing that he had to communicate certain information right away in order to save innocent lives," according to the lawyers.

The defense lawyers also said they had experts, including two former directors of the Information Security Oversight Office, which is responsible to the president for policy and oversight of the government-wide security classification system. The government had unsuccessfully sought to bar the testimony of J. William Leonard, the most recent former director.

Along with that of other experts, Leonard's testimony would "establish that the information was innocuous and that the defendants had every reason to believe that their conduct was innocent," the lawyers wrote. That was important because Ellis had ruled that the government would have to prove that Rosen and Weissman "disclosed the information knowing that their conduct was illegal, knowing the information was closely held, and knowing that the information was potentially damaging to national security," according to the lawyers' letter.